Short answer: E04 on an HLN diesel heater means the ECU has detected a short circuit on the flame sensor — the sensor's resistance reads too low. On 8kW rotary-switch models without a screen, the same fault is flashed as blink code 00100. A simple disconnect-and-rescan test separates a bad sensor from a bad ECU.

What E04 means

E04 is the mirror image of E00: where E00 is an open circuit (resistance too high), E04 is a short (resistance too low). On air heaters, a short-circuit reading is below 500 Ω — a sensor measuring, say, 486 Ω is shorted.

Factory diagnosis

Air heaters (Aerolyn 2kW / 4kW / 8kW)

1. Disconnect the flame sensor (the green connector) and scan for fault codes again.
2. If the code changes to E00 (open circuit — expected with the sensor unplugged), the short was in the sensor: replace the combo sensor.
3. If E04 is still displayed with the sensor disconnected, the short is not in the sensor — test the ECU and replace it if necessary.

Coolant heaters (Aquano 5K / 10K / 12K / 15K)

1. Check the wiring to the flame sensor for shorts (chafed insulation, pinched wires, moisture in connectors).
2. Replace the flame sensor, or the ECU if the fault remains.

FAQ

Why does E00 appearing during the test mean the sensor is bad?
With the sensor unplugged, the ECU should see an open circuit (E00). If it does, the ECU's measuring circuit works — so the short it saw before must have been in the sensor. If it still sees a short with nothing connected, the problem is on the ECU side.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E04 is 00100 — two short, one long, two short (long = 1, short = 0).

Is E04 the same on other heater brands?
No. Fault-code tables are brand-specific — this table applies to HLN heaters only.

All HLN fault codes in one place: HLN Diesel Heater Error Codes Explained (E01–E32). Source: HLN factory service manual (2026 revised editions).